Field of the Invention
The present invention pertains to a method for the selective dry etching of a first semi-conductive compound, which contains no aluminium molecules, without the attacking of a second semi-conductive compound comprising an aluminium molecule in its formula. This method is applied to the hollowing out of the recess in which is deposited the gate metallization of a transistor of the field effect type, made with materials of the III-V group, one of which has aluminum in its formula. The invention also pertains to the transistor made by this method.
It is known that the access or contact resistances between the (source, drain and gate) electrodes of a transistor and the active layer of this transistor plays a major role in determining the electrical characteristics of the transistor. This is why the source and drain metallizations are deposited on a layer of material which is a good conductor. But this layer must be hollowed out, between the source and the drain, if the gate metallization is to be as close as possible to the active layer of the transistor: thus, a depression is formed, generally known as a recess, between the source and the drain. The etching of the recess can be done according to wet or dry methods, but it must stop at the active layer.
The invention therefore, pertains to a method of dry, reactive, ion etching which is anisotropic and which is highly selective between two materials deposited on one support and stacked, of which one material, the one which is in contact with the support, comprises aluminum in a range of 10% to 40% while the other material, the one which is at the surface of the stack, contains no aluminum. To make the description clearer, the invention will be explained on the basis of an example of a transistor, the active layer of which is made of AlGaAs-Al.sub.x Ga.sub.l-x As more exactly, and the layer of contact between the source and the drain is made of GaAs, although the invention more generally pertains to pairs of materials of the group III-V, only one of which has aluminum. According to the invention, the layer of GaAs is selectively etched by reactive ion etching using a freon plasma with the formula CCl.sub.2 F.sub.2. In the plasma formed, the chlorine ions act with the gallium from the GaAs to give a compound of the GaCl.sub.y type which is volatile towards 100.degree. C., while the fluorine ions act with the aluminum from AlGaAS to give a compound of the AlF.sub.z type which is not volatile before about 1300.degree. and which, therefore, remains at the surface of the layer of AlGaAs, protecting it from attack at the etching temperature which must be less than about 130.degree. C. to be compatible with the masking resins. The pressure conditions of the plasma can be used to control the anisotropism of the etching and, especially, to make a sub-etching of the layer under the resin layer which protects it.